I am replacing my HVAC system. 2 story home with basement. Single zone. I have a 5-7 degree temperature variance between top and bottom floor. One vendor says to go up a ton and another says to go with muti speed fans. Any advice?

Furnaces and fan coils equipped with ECM blower motors are designed for economical continuous fan operation, which is very effective at minimizing temperature difference from level to level in my home. Oversizing the cooling system can lead to problems with humidity control, as the cooling cycles may become too short/brief to effectively dehumidify the indoor air.

Going up is not a good idea. The unit will overcool the house, take a long cycle time and raise the humidity. Depending on solar load, window construction, attic insulation etc, 5-7 degrees is not out of the question. Keeping the fan on, although costing more money, will bring it down to 1-2 degrees. If you have separate trunk risers, you could consider a zone valve with 2 t'stats.